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I've just learnt that a part of the photosynthesis reaction is the light reaction, where sunlight is absorbed by chloroplasts, which then excites electrons to higher energy levels, allowing for water molecules to be broken down.

My question is whether this can also happen in pools of water? I mean, if all that is really needed is sunlight, then I don't see why water molecules in pools won't get excited and thus break down too. If not, then how do chloroplasts assist in this? And on top of that, why don't chloroplasts get damaged since this is similar to how UV rays damage our skin as it can excite electrons in cells and damage genes?

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    $\begingroup$ Photolysis without a chloroplast is about as efficient as driving without a car. $\endgroup$ – Ivan Neretin Jul 8 at 14:53
  • $\begingroup$ You are missing the other half-reaction ($\ce{CO2}$ to carbs), and the catalysts. If you add some cyanobacteria to the pool, you are all set. That is where the oxygen in the atmosphere initially came from. $\endgroup$ – Karsten Theis Jul 8 at 15:19
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    $\begingroup$ Photolysis with a chloroplast is highly tuned, but not necessarily efficient (depending on how you define efficiency). However, sunlight is basically free if you can get your hands (leaves?) on it, so efficiency there is relative to other competing plants. $\endgroup$ – Zhe Jul 8 at 15:27
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    $\begingroup$ In the light reactions in photosynthesis the photon never interacts with the water molecule. The photon is absorbed by one of many chlorophyll molecules which transfer the energy to a chlorophyll dimer and, after a series of electron transfer steps, generates enough energy to extract an electron from water (or a similar fuel in photosynthetic bacteria). This latter process takes place in a protein as do all the reactions in photosynthesis. The reactions have been studied in detail by ultrafast spectroscopy. The structures of the proteins are known from x-ray diffraction. $\endgroup$ – porphyrin Jul 8 at 19:54
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Photosynthesis is a lot more complex than just breaking down water

Yes, photosynthesis involves a reaction which partly involves absorbing photons and breaking down water. But overall it is far more complex than that as it also involves absorbing carbon dioxide and turning that into sugars and other molecules that plant can use a fuel. Breaking down water is a byproduct of that larger process as is the oxygen released by it.

The detail of the reaction involved is complex and consists of dozens of individual steps. It isn't as simple as absorbing a photon and splitting water. In fact, there are two separate and individually complex systems of proteins involved in the water-splitting part of the process: photosystem I absorbs light and generates higher energy electrons; photosystem II also absorbs light, breaks water molecules part and generates higher energy electrons. But these processes are so complex that we don't fully understand how they work (and our understanding of the structure of photosystem II was updated as recently as 2014). And that machinery is very complex involving dozens of individual proteins and components.

Given the complexity of the processes behind water-splitting in photosynthesis (which involves multiple photons and electrons) it should be clear that there is no easy way to perform the reaction.

You don't just need photons and water molecules. You need some of the most complex molecular machinery we know of and multiple photons. Sunlight + water isn't going to achieve it.

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